1. Technical Field
This invention relates to electronic still cameras useful in professional or amateur still photography and which use a semiconductor imaging sensor--such as a charge coupled device imager--instead of film.
2. Description of Related Art
An example of an electronic still camera is the Canon RC-701 still video camera recently introduced for commercial sale by Canon Inc., 3-30-2 Shimomaruko, Otako, Tokyo 146 JAPAN. Such cameras typically include a charge coupled device (CCD) imager for sensing an image, and a magnetic disk recording system for storing the image (instead of film). Such imagers suffer from internally generated noise, such as the so-called "dark current" which accumulates in each individual CCD "bucket" even in the absence of any incident light on the imager. This dark current diminishes the ability of the imager to clearly sense an image under low light or nighttime conditions, thus reducing the effective sensitivity of the imager. This necessitates the use of a flash attachment or the like for nighttime photography. Of course, a flash attachment suffers from the disadvantage that the flash illumination falls off with the square of the distance between the flash attachment and the subject being photographed.
The dark current of a CCD imager varies with temperature. In fact, various signal processing techniques are known for compensating for temperature variations in a video signal generated by a charge coupled device imager. (See, for example, Sequin et al, Charge Coupled Devices, Academic Press, 1973, and U.S. Pat. No. 4,307,423.) A suggestion has been made to reduce noise in a solid state television camera, comprising a solid state line scanning imager, by cooling the solid state line scanning device with a thermo-electric module. (See U.S. Pat. No. 4,532,544.) However, no suggestion is made for intelligently controlling the imager temperature. Therefore, it would not seem that such a television camera would be useful under a wide range of frequently changing exposure conditions or settings, such as may be encountered in still photography where aperture size and shutter speed are sometimes changed for every exposure.
3. The Problem
The problem is how to adapt a CCD imager to high-quality still photography. In such applications, there is a need to precisely optimize aperture size and exposure time (and imager sensitivity if possible) for each individual exposure, since subject brightness, ambient lighting, etc., may be different for each exposure. There is a need to accommodate daylight as well as nighttime conditions. The prior art solution of simply using a flash attachment for nighttime photography provides insufficient illumination at long range. At short range, a flashbulb can disturb the subject being photographed (or damage it, in the case of indoor close-up photography of priceless art objects, for example).